The assembly comprising the motor unit (9), the drive pulley (8), the driven pulley (6), and the grooved wheel (3) is pivotally mounted relative to the fixed position pressure wheel (2) by means of an actuator (19) whose piston (21) presses against the frame (1) under the control of a slide valve (20) whose slide (27) is servo controlled to compensate variations from a constant value (L) for the distance between the axis (10) of the motor unit (9) and the axis (11) of the driven pulley (6), whereby the pressure exerted on the wire automatically adjusts itself to the required value.